It is well known in the art to provide protective wraps for protecting tubular articles such as pipelines and the like. Such protection is necessitated by both external and internal degradative forces exerted on in-ground pipelines. External forces are exerted by the environment and manifest themselves in soil stresses, sedimentation, water accumulation, bacteria accumulation, etc.; all assisting in destroying the integrity of in-ground pipelines and the like. Internal forces are primarily stresses due to the segmented nature in which pipelines are laid in the ground.
As is common in the art, a pipeline consists of a series of individual pipe sections in which adjacent pipe sections are welded together to form what are referred to simply as pipe joints. After joining, the pipe joints are typically protected from the aforementioned degradative forces by an adhesive layer and an outer liquid-impermeable wrap.
A particularly efficacious outer wrap comprises a so-called shrink sleeve or heat-shrinkable film which is applied over the pipe joint and then heated to cause shrinkage of the film so that it and the underlyiny adhesive layer adhere tightly to the pipe joint.
The adhesive may be contained initially on the heat-shrinkable film backing to form a heat-shrinkable adhesive tape. Alternatively, and in lieu of having the adhesive on heat-shrinkable backing material, it is also known to first apply a coating of adhesive to the pipes surface and thereafter apply a heat-shrinkable film thereover.
While not intended to be construed to be a comprehensive survey of the art, the following patents are nevertheless considered to be illustrative of heat-shrinkable films.
U.S. Pat. No. 3,144,398 relates to the preparation of irradiated polyethylene which can be readily changed from a stretched to a shrunken condition. The objectives are said to be accomplished by cold stretching the polyethylene at a temperature of 65.degree. C., and then irradiating at a specified dosage, preferably with electrons.
U.S. Pat. No. 3,144,399 relates to an irradiated, biaxially oriented polyethylene stretched at least 100% in each direction and below the break limit. The polyethylene is first irradiated at a specified dosage, then heated to or beyond its transparent point, and stretched to orient. The stretched condition is maintained while cooling to room temperature.
U.S. Pat. No. 3,455,337 relates to a differentially irradiated crosslinked polymeric recoverable article, tubing being particularly disclosed, characterized by containing a sufficient crosslink density near one surface to be substantially infusible and containing a gradual decreasing density of cross-linking throughout its thickness, the opposed surface being substantially non-crosslinked. As is described therein, the article is formed by first subjecting it to a dosage of irradiation insufficient to render the opposed surface infusible. Once the tubing has been differentially crosslinked, it is heated and subjected to differential pressure between the inside and the outside, the pressure being sufficient to cause the tube to expand in a controlled fashion.
U.S. Pat. No. 3,886,056 has for its objective to prepare from polyethylene having a high crystallinity a polyethylene having highly raised melting and softening temperatures, improved transparency and excellent dimensional stability at high temperatures. This objective is said to be accomplished by irradiating with a dosage of 0.2-16 Megarads to produce a crosslinked polyethylene having a gel content of at least one weight percent; extending the crosslinked polymer in at least one direction at a temperature of at least the anisotropic melting point; and then cooling.
U.S. Pat. No. 3,949,110 discloses a method of making a heat shrinkable tubing including the steps of irradiating the tube, heating to at least the softening temperature, partially inflating, and then thermosetting by cooling.
U.S. Pat. No. 3,988,399 relates to heat recoverable wraparound sleeves for pipejoints, cables, wire splices and the like which possess the ability to curl in involute fashion upon the application of heat. As is disclosed, for example, in Col. 6, one or both of the primary exterior faces can be coated with a suitable adhesive.
U.S. Pat. No. 4,348,438 discloses a process for preparing shrink wraps from a homopolymer of ethylene or a copolymer of ethylene with up to 20 weight percent of one or more other monomers. As disclosed, the film is uniaxially cold-oriented at a temperature of at least 5.degree. C. below the film's melting point, irradiated with ionizing radiation and sealed along a seam running perpendicular to the direction of orientation. The irradiation, which is preferably carried out before the orientation, is at a dosage of 1-20 Megarads, about 3 to 5 Megarads being stated to be preferred.
U.S. Pat. No. 4,469,742 relates to a multilayer cook-in shrink film comprising: (1) a specified sealing layer; (2) a shrink layer; (3) a specified adhesive layer; (4) a barrier layer; (5) another adhesive layer; and (6) and a base layer, the respective layers being melt bonded and irradiated to crosslink sufficient to resist delamination during use.
U.S. Pat. No. 4,517,234 relates to a flat length of heat recoverable material having integral latching means so that the material can be wrapped around cable, pipe, etc., latched and then shrunk.
U.S. Pat. No. 4,521,470 relates to a system for enclosing an object by installing consecutively or as a single article: (a) a heat-softenable adhesive; (b) a specified thermoplastic polymeric material; and (c) a heat-recoverable cover, and thereafter heating to cause recovery (shrink).
U.S. Pat. No. 4,590,020 teaches an oriented high density polyethylene film having maximum crosslinking at the surfaces and minimum crosslinking inwardly. The film is prepared by crosslinking opposed surfaces by irradiation with electron rays in such a manner that the degree of crosslinking decreases from the surfaces inwardly so that the outer layer portions have gel fraction between 0-5%; and thereafter heating and stretching to orient and produce a film between 10-50 microns. As stated in Col. 3, both sides should be irradiated with the same dosage, the penetrating power of the electron rays being properly adjusted according to the thickness by changing the applied voltage or by using a shield.
My copending applications Ser. No. 003,091 filed Jan. 14, 1987 and 1770,177 filed Mar. 18, 1988 also relate to heat shrinkable films and tapes which may be employed as pipewraps, e.g. for protecting joints of pipelines. The disclosures of these copending applications which are not necessary for a clear understanding of the present invention, are incorporated by reference herein.
After the shrink sleeve is positioned over the pipejoint, means must be provided to retain it in place on the underlying pipe surface prior to heating. Various means for joining the open longitudinal seam of a heat-recoverable shrink-wrap during installation of tubular articles such as pipes are also well known in the art. The task to be met by such closures is primarily to maintain the seam against the internal shrinkage forces of the sleeve during its heat recovery when heat fused to the underlying surface such as pipe coating.
First among the known closure techniques are mechanical closures such as metal slip-on channels, disclosed in U.S. Pat. No. 3,455,335; buttons, as disclosed in U.S. Pat. No. 3,379,218; threaded loops, as disclosed in U.S. Pat. No. 3,530,383; and embedded inserts, as disclosed in U.S. Pat. No. 3,542,077. While mechanical closures have found commercial application, they do possess disadvantages. Among the disadvantages is the expense involved in manufacturing metal closures and in applying such closures to large diameter objects often, as with pipelines, in field stations. Thermal stresses which may be generated during the heat-recovery stage, are also disadvantageous.
While presently known film adhesive closures eliminate disadvantages exhibited by mechanical closures, they are subject to their own disadvantages. Exemplary of adhesive closure materials are the cyano acrylates as disclosed in U.S. Pat. No. 3,959,052, silicone adhesives, as disclosed in U.S. Pat. No. 4,153,747; chloroprene-based contact adhesives, as disclosed in U.S. Pat. No. 3,770,556, and crosslinked hot-melt adhesives as described in U.S. Pat. No. 4,220,676.
While hot melt adhesive closure systems are in commerce they are problematic since a cool portion must be maintained along the longitudinal axis of the sleeve to prevent the forces of contraction during application of heat from shearing the hot melt adhesive under the closure and thereby opening the closure. In other words, since hot melt adhesive closures have a lower melting point than the shrinking point of the polyolefin sleeve, the portion lying directly under the hot melt patch cannot be shrunk without liquidifying the hot melt adhesive and causing the polyolefin sleeve to pull apart from the hot melt patch thereby destroying the closure.
Pressure sensitive adhesives have been disclosed in U.S. Pat. No. 4,153,747 and 4,268,559. The pressure sensitive adhesive closures are either crosslinked or non-crosslinked. The disadvantage encountered with the crosslinked pressure sensitive adhesive closures again is liquification of the adhesive resulting in the pulling apart of the underlying polyolefin sleeve and thereby destroying the closure. This problem is partially overcome by using partially oriented sleeves, sleeves wherein the edges are unstretched. The inherent disadvantage of such a system lies in the necessity to manufacture different size sleeves to accommodate the varying pipe diameter sizes.
Crosslinked pressure sensitive adhesive closures provide a better bond between the closure and the sleeve because the crosslinking allows for shear strength at high temperatures. However, while the problem is solved in the overlying layers, the problem is not addressed in the bond between the substrate and the sleeve. The sleeves are still applied with a mastic or a hot melt adhesive. Consequently upon heating, the underlying bond is destroyed. Exemplary of problems encountered by crosslinked pressure sensitive adhesives, is U.S. Pat. No. 4,731,274 of Bonk et al disclosing a closure which employes two adhesives. Namely, one adhesive secures attachment of the closure to the substrate, a heat-activatable adhesive, and a second adhesive serves as a closing means for the closure device itself, a pressure sensitive adhesive. Since the two adhesives have different melting points, upon the application of heat, the heat-activatable adhesive will become liquified and sever the attachment between the closure and the substrate.
Alternatively, Bonk et al, suggests separating the pressure sensitive adhesive from the heat-activatable adhesive by graft-polymerizing the entire polyolefin sheet so as to allow for the attachment of a separate closure means over the sleeve lap. Notably, graft-polymerization of the entire sleeve, is a very costly and labor intensive process.
Lastly, and in general pressure sensitive adhesives are more permeable to oxygen and water than hot melt adhesives or polyolefins, a disadvantage which is very detrimental when applied to in-ground pipes whose integrity depends on the inhibition of degradative environmental forces.
The fourth known closure mechanism is a "Canusa wrapid" sleeve and involves a single unit sleeve. This sleeve has an adhesive free area which allows for direct fusion between the substrate pipewrap and the sleeve. The problem encountered by this closure system again is the necessity for having a large inventory to accommodate the varying pipe diameter sizes.
The task of the present invention, given the aforementioned disadvantages, simply stated, is to provide a novel closure to cover the lap of a heat-shrinkable film or tape such as are heretofore known in the art, which closure resists the shrink forces of the wrap, thereby allowing for tight closure and consequently offer protection against corrosion caused by the external and internal forces previously described.